Electromagnetic drives based on the known physical phenomena of the Lorentz force or reluctance force are used for a wide variety of actuating tasks. With the aid of electrical coils and magnetic materials, the interaction between electrical and magnetic fields and a force action that is generated are utilized such that the electrical energy is ultimately converted into mechanical energy. In accordance with this principle, either rotational or linear (translational) movements can be performed.
Electromagnetic drives are used inter alia in projection exposure apparatuses for semiconductor lithography for the purposes of mechanically influencing or manipulating or deforming optical elements in the illumination systems thereof, for example in order to control the beam path of a radiation source.
In this regard, it is known from WO 2005/026801 A2 to adjust optical elements, such as mirrors, for EUV projection exposure apparatuses in multiple degrees of freedom using driveable movement axes via Lorentz actuators. Plunger coil actuators can be used for this purpose, wherein a linearly moveable actuating element (translator) in the form of a magnet can be mechanically influenced by electromagnetic interaction with a statically mounted coil surrounding the translator. In this case, the translator is connected to the optical element, to which a movement carried out can be directly transmitted. The coil itself, which in this embodiment is in the form of a stator, is in the case of such an arrangement connected via a stator or coil holder to surrounding structures, for example a housing part.
An issue in the case of an electromagnetic drive of the type having a coil and a coil holder is that the coil holder can also inductively interact with the coil. The current flow in the coil generates a magnetic field which, aside from the desired effect on the translator or rotor, additionally induces parasitic eddy currents in nearby conductive materials such as the coil holder. The abovementioned eddy currents then likewise generate a magnetic field, which opposes the original field in the coil. In this way, the efficiency of the energy conversion of the drive, or a force action on the translator or the rotor, is reduced. In particular in the case of high frequencies, control errors can thus arise in a control loop, to the point of instability of the system, owing to lags of the manipulated variable. A similar issue also arises if, in an alternative embodiment, the actuating element is formed as a coil and the stator is formed as a magnet, or both devices are formed as coils.
To prevent or reduce an undesired induction of eddy currents in the coil holder, it is known to use a non-conductive material to form the coil holder. However, in numerous applications, it is desirable for heat losses that arise in the coil to be dissipated, via adequate thermal conductivity of the coil holder, in order to lengthen the service life of the coil. It is known that materials with good thermal conductivity normally likewise exhibit good electrical conductivity. Ceramics, which are poor electrical conductors but good thermal conductors, are an exception to this. The processing of ceramics is however technically complex and leads to considerably greater component tolerances than, for example, the metal processing methods that are customary for metals.
A further approach for reducing undesired induced eddy currents involves shortening the long electrically conductive path in the coil holder via electrically insulating slots. The coil holder is thus divided into relatively small segments, such that a continuous current flow through the entire component can no longer form. However, in this approach, eddy currents are induced locally in the individual segments, and the adverse effect can thus be lessened only to a limited extent.
As an alternative to the abovementioned concepts, a so-called vibrating coil overhang is known. Here, the coil is extended such that the coil holder can be mounted at an adequately great distance from the translator or rotor. It is thereby sought to prevent or reduce a disruptive force action of the parasitic magnetic field originating from the coil holder on the translator or rotor. But issues relating to this approach are an enlarged structural form, a possibly increased transmission of vibrations owing to a lever action, and increased electrical losses owing to the enlarged coil, which can also adversely affect the service life owing to intensified heat generation.